Mobility management is a critical aspect of any wireless communication system, from the early analog systems to modern digital systems such as GSM, 3GPP, Long Term Evolution (LTE), and the like. Fundamental to mobility management is the ability of a user to move geographically while maintaining operative connectivity with the system. To accomplish this, the user's mobile terminal—referred to herein by the more general term User Equipment (UE)—establishes service with successive serving base stations (also known as Node B) as the user travels. The process of transferring service from one serving base station to another is known as handoff (HO).
The decision for HO of a UE from a current (serving) base station to another one is generally made by the network. Whether or not to hand over a UE is based primarily on comparing either the received signal strength (RSS), or the signal to interference plus noise ratio (SINR), or a combination of both, of signals received from two or more base stations. RSS is usually measured over pilot signals transmitted by a base station.
A number of parameters are considered in the comparison. For example, the RSS of an adjacent base station should exceed the RSS of the current serving base station by a predetermined margin, and for a predetermined duration. This helps to reduce the “ping-pong effect” of a UE being handed back and forth between two base stations when positioned where their corresponding cells overlap. A “handover prohibit timer,” which defines the minimum time duration between two consecutive handovers, also mitigates this effect. These parameters typically are maintained at the base station (or elsewhere in the network), which makes the HO decision.
A number of HO-related parameters are maintained and utilized by the UE. For example, while the UE may make RSS measurements often, to reduce signaling these measurements are not all reported to the base station. Only when a measurement exceeds a threshold for a predetermined duration, for example, would it be reported. Both the threshold value and the corresponding “Time To Trigger” (TTT) duration are HO parameters with which the UE is provisioned. The RSS, or other measured value, may be filtered over a measurement window, to reduce transient effects, prior to reporting the value. The duration of the measurement window, and a parameter related to the weight to be given prior measured values, are additional HO-related parameters.
HO is performed when the UE is in active mode. A similar procedure, cell reselection, is carried out by the UE in idle mode. Several parameters associated with cell reselection are broadcast by the network to UEs, such as a serving cell strength or/and quality threshold, a factor indicating how often the UE should evaluate cell reselection criteria, and a hysteresis value.
In general, the optimal values of various measurement, HO, and HO-related parameters depend heavily on the speed of the UE. For example, a parameter specifying the duration over which a factor such as RSS should be measured may optimally be relatively long for a slow-moving UE, to filter the effects of fast fading. However, a fast-moving UE would optimally measure the RSS over a shorter duration since the channel experienced by the UE may be changing rapidly. Conforming the HO parameters to the UE speed would result in more accurate HO performance—i.e., fewer unnecessary HOs. One way to determine the speed of a UE is by measuring the Doppler shift in its signal carrier frequency. This requires the UE to be in active mode to estimate speed, leading to poor HO parameter selection performance in idle mode or active mode employing discontinuous reception (DRX). The UE speed may alternatively be measured at the base station. This requires increased signaling overhead to frequently transmit updated HO parameters to the UE. In either case, an error in UE speed measurement that leads to inappropriate adaptation of HO parameters will reduce HO performance by generating unnecessary HO events.